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[PATCH v5 12/12] s/recommendable/recommended 486+ messages / 3 participants [nested] [flat]
* [PATCH v3 11/12] s/recommendable/recommended @ 2019-05-10 02:22 Justin Pryzby <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Justin Pryzby @ 2019-05-10 02:22 UTC (permalink / raw) --- doc/src/sgml/btree.sgml | 2 +- doc/src/sgml/libpq.sgml | 2 +- doc/src/sgml/runtime.sgml | 2 +- 3 files changed, 3 insertions(+), 3 deletions(-) diff --git a/doc/src/sgml/btree.sgml b/doc/src/sgml/btree.sgml index 996932e..283db7f 100644 --- a/doc/src/sgml/btree.sgml +++ b/doc/src/sgml/btree.sgml @@ -60,7 +60,7 @@ contain the single-type operators (and associated support functions) for its input data type, while cross-type comparison operators and support functions are <quote>loose</quote> in the family. It is - recommendable that a complete set of cross-type operators be included + recommended that a complete set of cross-type operators be included in the family, thus ensuring that the planner can represent any comparison conditions that it deduces from transitivity. </para> diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml index 8a8427f..4b031ff 100644 --- a/doc/src/sgml/libpq.sgml +++ b/doc/src/sgml/libpq.sgml @@ -7107,7 +7107,7 @@ int PQresultSetInstanceData(PGresult *res, PGEventProc proc, void *data); Beware that any storage represented by <parameter>data</parameter> will not be accounted for by <function>PQresultMemorySize</function>, unless it is allocated using <function>PQresultAlloc</function>. - (Doing so is recommendable because it eliminates the need to free + (Doing so is recommended because it eliminates the need to free such storage explicitly when the result is destroyed.) </para> </listitem> diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml index 798da30..21a7ce3 100644 --- a/doc/src/sgml/runtime.sgml +++ b/doc/src/sgml/runtime.sgml @@ -111,7 +111,7 @@ <command>initdb</command> will attempt to create the directory you specify if it does not already exist. Of course, this will fail if <command>initdb</command> does not have permissions to write in the - parent directory. It's generally recommendable that the + parent directory. It's generally recommended that the <productname>PostgreSQL</productname> user own not just the data directory but its parent directory as well, so that this should not be a problem. If the desired parent directory doesn't exist either, -- 2.7.4 --cWoXeonUoKmBZSoM Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="v3-0012-Cleanup-remove-update-references-to-OID-column.patch" ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH v5 12/12] s/recommendable/recommended @ 2019-05-10 02:22 Justin Pryzby <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Justin Pryzby @ 2019-05-10 02:22 UTC (permalink / raw) --- doc/src/sgml/btree.sgml | 2 +- doc/src/sgml/libpq.sgml | 2 +- doc/src/sgml/runtime.sgml | 2 +- 3 files changed, 3 insertions(+), 3 deletions(-) diff --git a/doc/src/sgml/btree.sgml b/doc/src/sgml/btree.sgml index 5881ea5..b0e0f08 100644 --- a/doc/src/sgml/btree.sgml +++ b/doc/src/sgml/btree.sgml @@ -60,7 +60,7 @@ contain the single-type operators (and associated support functions) for its input data type, while cross-type comparison operators and support functions are <quote>loose</quote> in the family. It is - recommendable that a complete set of cross-type operators be included + recommended that a complete set of cross-type operators be included in the family, thus ensuring that the planner can represent any comparison conditions that it deduces from transitivity. </para> diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml index 8a8427f..4b031ff 100644 --- a/doc/src/sgml/libpq.sgml +++ b/doc/src/sgml/libpq.sgml @@ -7107,7 +7107,7 @@ int PQresultSetInstanceData(PGresult *res, PGEventProc proc, void *data); Beware that any storage represented by <parameter>data</parameter> will not be accounted for by <function>PQresultMemorySize</function>, unless it is allocated using <function>PQresultAlloc</function>. - (Doing so is recommendable because it eliminates the need to free + (Doing so is recommended because it eliminates the need to free such storage explicitly when the result is destroyed.) </para> </listitem> diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml index ecdaafc..e3d0dec 100644 --- a/doc/src/sgml/runtime.sgml +++ b/doc/src/sgml/runtime.sgml @@ -111,7 +111,7 @@ <command>initdb</command> will attempt to create the directory you specify if it does not already exist. Of course, this will fail if <command>initdb</command> does not have permissions to write in the - parent directory. It's generally recommendable that the + parent directory. It's generally recommended that the <productname>PostgreSQL</productname> user own not just the data directory but its parent directory as well, so that this should not be a problem. If the desired parent directory doesn't exist either, -- 2.7.4 --FkmkrVfFsRoUs1wW-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH v22 6/8] Row pattern recognition patch (docs). @ 2024-09-19 04:48 Tatsuo Ishii <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Tatsuo Ishii @ 2024-09-19 04:48 UTC (permalink / raw) --- doc/src/sgml/advanced.sgml | 82 ++++++++++++++++++++++++++++++++++++ doc/src/sgml/func.sgml | 54 ++++++++++++++++++++++++ doc/src/sgml/ref/select.sgml | 38 ++++++++++++++++- 3 files changed, 172 insertions(+), 2 deletions(-) diff --git a/doc/src/sgml/advanced.sgml b/doc/src/sgml/advanced.sgml index 755c9f1485..b0b1d1c51e 100644 --- a/doc/src/sgml/advanced.sgml +++ b/doc/src/sgml/advanced.sgml @@ -537,6 +537,88 @@ WHERE pos < 3; <literal>rank</literal> less than 3. </para> + <para> + Row pattern common syntax can be used to perform row pattern recognition + in a query. The row pattern common syntax includes two sub + clauses: <literal>DEFINE</literal> + and <literal>PATTERN</literal>. <literal>DEFINE</literal> defines + definition variables along with an expression. The expression must be a + logical expression, which means it must + return <literal>TRUE</literal>, <literal>FALSE</literal> + or <literal>NULL</literal>. The expression may comprise column references + and functions. Window functions, aggregate functions and subqueries are + not allowed. An example of <literal>DEFINE</literal> is as follows. + +<programlisting> +DEFINE + LOWPRICE AS price <= 100, + UP AS price > PREV(price), + DOWN AS price < PREV(price) +</programlisting> + + Note that <function>PREV</function> returns the price column in the + previous row if it's called in a context of row pattern recognition. Thus in + the second line the definition variable "UP" is <literal>TRUE</literal> + when the price column in the current row is greater than the price column + in the previous row. Likewise, "DOWN" is <literal>TRUE</literal> when when + the price column in the current row is lower than the price column in the + previous row. + </para> + <para> + Once <literal>DEFINE</literal> exists, <literal>PATTERN</literal> can be + used. <literal>PATTERN</literal> defines a sequence of rows that satisfies + certain conditions. For example following <literal>PATTERN</literal> + defines that a row starts with the condition "LOWPRICE", then one or more + rows satisfy "UP" and finally one or more rows satisfy "DOWN". Note that + "+" means one or more matches. Also you can use "*", which means zero or + more matches. If a sequence of rows which satisfies the PATTERN is found, + in the starting row of the sequence of rows all window functions and + aggregates are shown in the target list. Note that aggregations only look + into the matched rows, rather than whole frame. On the second or + subsequent rows all window functions are NULL. Aggregates are NULL or 0 + (count case) depending on its aggregation definition. For rows that do not + match on the PATTERN, all window functions and aggregates are shown AS + NULL too, except count showing 0. This is because the rows do not match, + thus they are in an empty frame. Example of a <literal>SELECT</literal> + using the <literal>DEFINE</literal> and <literal>PATTERN</literal> clause + is as follows. + +<programlisting> +SELECT company, tdate, price, + first_value(price) OVER w, + max(price) OVER w, + count(price) OVER w +FROM stock + WINDOW w AS ( + PARTITION BY company + ORDER BY tdate + ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING + AFTER MATCH SKIP PAST LAST ROW + INITIAL + PATTERN (LOWPRICE UP+ DOWN+) + DEFINE + LOWPRICE AS price <= 100, + UP AS price > PREV(price), + DOWN AS price < PREV(price) +); +</programlisting> +<screen> + company | tdate | price | first_value | max | count +----------+------------+-------+-------------+-----+------- + company1 | 2023-07-01 | 100 | 100 | 200 | 4 + company1 | 2023-07-02 | 200 | | | 0 + company1 | 2023-07-03 | 150 | | | 0 + company1 | 2023-07-04 | 140 | | | 0 + company1 | 2023-07-05 | 150 | | | 0 + company1 | 2023-07-06 | 90 | 90 | 130 | 4 + company1 | 2023-07-07 | 110 | | | 0 + company1 | 2023-07-08 | 130 | | | 0 + company1 | 2023-07-09 | 120 | | | 0 + company1 | 2023-07-10 | 130 | | | 0 +(10 rows) +</screen> + </para> + <para> When a query involves multiple window functions, it is possible to write out each one with a separate <literal>OVER</literal> clause, but this is diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index 6f75bd0c7d..4482c80a70 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -23261,6 +23261,7 @@ SELECT count(*) FROM sometable; returns <literal>NULL</literal> if there is no such row. </para></entry> </row> + </tbody> </tgroup> </table> @@ -23300,6 +23301,59 @@ SELECT count(*) FROM sometable; Other frame specifications can be used to obtain other effects. </para> + <para> + Row pattern recognition navigation functions are listed in + <xref linkend="functions-rpr-navigation-table"/>. These functions + can be used to describe DEFINE clause of Row pattern recognition. + </para> + + <table id="functions-rpr-navigation-table"> + <title>Row Pattern Navigation Functions</title> + <tgroup cols="1"> + <thead> + <row> + <entry role="func_table_entry"><para role="func_signature"> + Function + </para> + <para> + Description + </para></entry> + </row> + </thead> + + <tbody> + <row> + <entry role="func_table_entry"><para role="func_signature"> + <indexterm> + <primary>prev</primary> + </indexterm> + <function>prev</function> ( <parameter>value</parameter> <type>anyelement</type> ) + <returnvalue>anyelement</returnvalue> + </para> + <para> + Returns the column value at the previous row; + returns NULL if there is no previous row in the window frame. + </para></entry> + </row> + + <row> + <entry role="func_table_entry"><para role="func_signature"> + <indexterm> + <primary>next</primary> + </indexterm> + <function>next</function> ( <parameter>value</parameter> <type>anyelement</type> ) + <returnvalue>anyelement</returnvalue> + </para> + <para> + Returns the column value at the next row; + returns NULL if there is no next row in the window frame. + </para></entry> + </row> + + </tbody> + </tgroup> + </table> + <note> <para> The SQL standard defines a <literal>RESPECT NULLS</literal> or diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml index d7089eac0b..7e1c9989ba 100644 --- a/doc/src/sgml/ref/select.sgml +++ b/doc/src/sgml/ref/select.sgml @@ -969,8 +969,8 @@ WINDOW <replaceable class="parameter">window_name</replaceable> AS ( <replaceabl The <replaceable class="parameter">frame_clause</replaceable> can be one of <synopsis> -{ RANGE | ROWS | GROUPS } <replaceable>frame_start</replaceable> [ <replaceable>frame_exclusion</replaceable> ] -{ RANGE | ROWS | GROUPS } BETWEEN <replaceable>frame_start</replaceable> AND <replaceable>frame_end</replaceable> [ <replaceable>frame_exclusion</replaceable> ] +{ RANGE | ROWS | GROUPS } <replaceable>frame_start</replaceable> [ <replaceable>frame_exclusion</replaceable> ] [row_pattern_common_syntax] +{ RANGE | ROWS | GROUPS } BETWEEN <replaceable>frame_start</replaceable> AND <replaceable>frame_end</replaceable> [ <replaceable>frame_exclusion</replaceable> ] [row_pattern_common_syntax] </synopsis> where <replaceable>frame_start</replaceable> @@ -1077,6 +1077,40 @@ EXCLUDE NO OTHERS a given peer group will be in the frame or excluded from it. </para> + <para> + The + optional <replaceable class="parameter">row_pattern_common_syntax</replaceable> + defines the <firstterm>row pattern recognition condition</firstterm> for + this + window. <replaceable class="parameter">row_pattern_common_syntax</replaceable> + includes following subclauses. <literal>AFTER MATCH SKIP PAST LAST + ROW</literal> or <literal>AFTER MATCH SKIP TO NEXT ROW</literal> controls + how to proceed to next row position after a match + found. With <literal>AFTER MATCH SKIP PAST LAST ROW</literal> (the + default) next row position is next to the last row of previous match. On + the other hand, with <literal>AFTER MATCH SKIP TO NEXT ROW</literal> next + row position is always next to the last row of previous + match. <literal>DEFINE</literal> defines definition variables along with a + boolean expression. <literal>PATTERN</literal> defines a sequence of rows + that satisfies certain conditions using variables defined + in <literal>DEFINE</literal> clause. If the variable is not defined in + the <literal>DEFINE</literal> clause, it is implicitly assumed + following is defined in the <literal>DEFINE</literal> clause. + +<synopsis> +<literal>variable_name</literal> AS TRUE +</synopsis> + + Note that the maximu number of variables defined + in <literal>DEFINE</literal> clause is 26. + +<synopsis> +[ AFTER MATCH SKIP PAST LAST ROW | AFTER MATCH SKIP TO NEXT ROW ] +PATTERN <replaceable class="parameter">pattern_variable_name</replaceable>[+] [, ...] +DEFINE <replaceable class="parameter">definition_varible_name</replaceable> AS <replaceable class="parameter">expression</replaceable> [, ...] +</synopsis> + </para> + <para> The purpose of a <literal>WINDOW</literal> clause is to specify the behavior of <firstterm>window functions</firstterm> appearing in the query's -- 2.25.1 ----Next_Part(Thu_Sep_19_13_59_47_2024_608)-- Content-Type: Text/X-Patch; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline; filename="v22-0007-Row-pattern-recognition-patch-tests.patch" ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
* [PATCH 1/2] Demonstrate possible race conditions in logical decoding. @ 2026-01-19 11:54 Antonin Houska <[email protected]> 0 siblings, 0 replies; 486+ messages in thread From: Antonin Houska @ 2026-01-19 11:54 UTC (permalink / raw) The problem is that the snapshot builder can create a snapshot before CLOG has been updated. That breaks visibility check that use such snapshot. For more details, see startup_race.spec. Success of the test means that the problem is present. Thus it would need to be modified if it should be merged into the tree. Another problem that currently prevents this test from being merged is that it hard-wires the logical decoding setup into the SET TRANSACTION command. I tried to modify the isolation tester so it can use the logical replication protocol (in which case the test could use the "CREATE_REPLICATION_SNAPSHOT ... (SNAPSHOT 'use')" command), but the tester does things that are not compatible with that protocol (e.g. it sets the application_name parameter). --- .../test_decoding/expected/startup_race.out | 85 ++++++++++++ contrib/test_decoding/specs/startup_race.spec | 126 ++++++++++++++++++ src/backend/access/transam/xact.c | 6 + src/backend/replication/logical/snapbuild.c | 3 + src/backend/utils/time/snapmgr.c | 66 +++++++++ src/include/utils/snapmgr.h | 1 + 6 files changed, 287 insertions(+) create mode 100644 contrib/test_decoding/expected/startup_race.out create mode 100644 contrib/test_decoding/specs/startup_race.spec diff --git a/contrib/test_decoding/expected/startup_race.out b/contrib/test_decoding/expected/startup_race.out new file mode 100644 index 00000000000..597fa617831 --- /dev/null +++ b/contrib/test_decoding/expected/startup_race.out @@ -0,0 +1,85 @@ +Parsed test spec with 6 sessions + +starting permutation: s1_assign_xid s2_set_snapshot s3_assign_xid s1_rollback s3_rollback s4_do_changes s5_wake_up_full_snapshot s2_scan s2_rollback s5_wake_up_before_clog s6_check +injection_points_attach +----------------------- + +(1 row) + +injection_points_attach +----------------------- + +(1 row) + +step s1_assign_xid: + BEGIN; + CREATE TABLE b(i int); + +step s2_set_snapshot: + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; + <waiting ...> +step s3_assign_xid: + BEGIN; + CREATE TABLE c(i int); + +step s1_rollback: + ROLLBACK; + +step s3_rollback: + ROLLBACK; + +step s4_do_changes: + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; + <waiting ...> +step s5_wake_up_full_snapshot: + SELECT injection_points_wakeup('snapbuild-full-snapshot'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s2_set_snapshot: <... completed> +step s2_scan: + TABLE a; + +i|j +-+- +1|1 +2|2 +(2 rows) + +step s2_rollback: + ROLLBACK; + +step s5_wake_up_before_clog: + SELECT injection_points_wakeup('before-clog-update'); + +injection_points_wakeup +----------------------- + +(1 row) + +step s4_do_changes: <... completed> +step s6_check: + SELECT * FROM a ORDER BY i; + +i|j +-+- +1|1 +2|2 +(2 rows) + +injection_points_detach +----------------------- + +(1 row) + +injection_points_detach +----------------------- + +(1 row) + diff --git a/contrib/test_decoding/specs/startup_race.spec b/contrib/test_decoding/specs/startup_race.spec new file mode 100644 index 00000000000..8f67e07fa7a --- /dev/null +++ b/contrib/test_decoding/specs/startup_race.spec @@ -0,0 +1,126 @@ +setup +{ + CREATE TABLE a(i int primary key, j int) WITH (autovacuum_enabled = off); + INSERT INTO a(i, j) VALUES (1, 1), (2, 2); + CREATE EXTENSION injection_points; +} + +session s1 +step s1_assign_xid +{ + BEGIN; + CREATE TABLE b(i int); +} +step s1_rollback +{ + ROLLBACK; +} + +session s2 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('snapbuild-full-snapshot', 'wait'); +} +# Use special, hard-wired snapshot name to set the initial snapshot from +# logical replication slot. +step s2_set_snapshot +{ + BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ; + SET TRANSACTION SNAPSHOT 'from_slot'; +} +# Perform the scan. +step s2_scan +{ + TABLE a; +} +step s2_rollback +{ + ROLLBACK; +} +teardown +{ + SELECT injection_points_detach('snapbuild-full-snapshot'); +} + +session s3 +step s3_assign_xid +{ + BEGIN; + CREATE TABLE c(i int); +} +step s3_rollback +{ + ROLLBACK; +} + +session s4 +setup +{ + SELECT injection_points_set_local(); + SELECT injection_points_attach('before-clog-update', 'wait'); +} +step s4_do_changes +{ + INSERT INTO a(i, j) VALUES (3, 3); + UPDATE a SET j = j + 1 WHERE i = 1; + DELETE FROM a WHERE i = 2; +} +teardown +{ + SELECT injection_points_detach('before-clog-update'); +} + +session s5 +step s5_wake_up_full_snapshot +{ + SELECT injection_points_wakeup('snapbuild-full-snapshot'); +} +step s5_wake_up_before_clog +{ + SELECT injection_points_wakeup('before-clog-update'); +} + +session s6 +step s6_check +{ + SELECT * FROM a ORDER BY i; +} + +permutation +# Let the snapshot builder go through all the states. The problematic case +# happens in the FULL_SNAPSHOT. +s1_assign_xid +# This should leave the builder in BUILDING_SNAPSHOT, waiting for the active +# transaction to end. +s2_set_snapshot +# Make sure that s1_rollback does not allow going to CONSISTENT directly. +s3_assign_xid +# Let the builder proceed to FULL_SNAPSHOT. It should stop at the injection +# point 'snapbuild-full-snapshot'. +s1_rollback +# The transaction of s3 is not needed anymore, CONSISTENT should be the next +# stage. +s3_rollback +# Perform data changes before the snapshot builder triggers creation of the +# RUNNING_XACTS record. This will stop before setting transaction status in +# CLOG. +s4_do_changes +# Unblock the injection point so that the snapshot can finally be created. +s5_wake_up_full_snapshot +# Use the snapshot for a scan. The snapshot will consider s4 not running +# anymore, however CLOG is not aware of the commit yet. Thus +# HeapTupleSatisfiesMVCC will consider the transaction aborted. In particular, +# for UPDATE, if both xmax of the old version and xmin of the new version are +# considered aborted, so the effects of the UPDATE are lost +# altogether. Similarly, INSERT and DELETE have no effect because the xmin +# transaction of the inserted tuple and xmax of the deleted tuple are +# considered aborted +s2_scan +s2_rollback +# CLOG can be updated now. +s5_wake_up_before_clog +# Scan the table again using a new transaction, with a normal transaction +# snapshot. The results are still wrong due to hint bits set incorrectly. +s6_check + diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c index c857e23552f..2fea45b2fed 100644 --- a/src/backend/access/transam/xact.c +++ b/src/backend/access/transam/xact.c @@ -65,6 +65,7 @@ #include "utils/builtins.h" #include "utils/combocid.h" #include "utils/guc.h" +#include "utils/injection_point.h" #include "utils/inval.h" #include "utils/memutils.h" #include "utils/relmapper.h" @@ -1348,6 +1349,9 @@ RecordTransactionCommit(void) &RelcacheInitFileInval); wrote_xlog = (XactLastRecEnd != 0); + /* Load the injection point before entering the critical section */ + INJECTION_POINT_LOAD("before-clog-update"); + /* * If we haven't been assigned an XID yet, we neither can, nor do we want * to write a COMMIT record. @@ -1514,6 +1518,8 @@ RecordTransactionCommit(void) { XLogFlush(XactLastRecEnd); + INJECTION_POINT_CACHED("before-clog-update", NULL); + /* * Now we may update the CLOG, if we wrote a COMMIT record above */ diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c index 7f79621b57e..9b09dc8eac1 100644 --- a/src/backend/replication/logical/snapbuild.c +++ b/src/backend/replication/logical/snapbuild.c @@ -141,6 +141,7 @@ #include "storage/procarray.h" #include "storage/standby.h" #include "utils/builtins.h" +#include "utils/injection_point.h" #include "utils/memutils.h" #include "utils/snapmgr.h" #include "utils/snapshot.h" @@ -1387,6 +1388,8 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn errdetail("Waiting for transactions (approximately %d) older than %u to end.", running->xcnt, running->nextXid)); + INJECTION_POINT("snapbuild-full-snapshot", NULL); + SnapBuildWaitSnapshot(running, running->nextXid); } diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 2e6197f5f35..f327b779004 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -110,10 +110,13 @@ #include "access/subtrans.h" #include "access/transam.h" #include "access/xact.h" +#include "access/xlogutils.h" #include "datatype/timestamp.h" #include "lib/pairingheap.h" #include "miscadmin.h" #include "port/pg_lfind.h" +#include "replication/logical.h" +#include "replication/snapbuild.h" #include "storage/fd.h" #include "storage/predicate.h" #include "storage/proc.h" @@ -1421,6 +1424,17 @@ ImportSnapshot(const char *idstr) (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ"))); + if (strcmp(idstr, "from_slot") == 0) + { + Snapshot snap; + + snap = create_test_snapshot(); + /* XXX sourcevxid shouldn't be needed in this special case */ + SetTransactionSnapshot(snap, NULL, MyProcPid, MyProc); + + return; + } + /* * Verify the identifier: only 0-9, A-F and hyphens are allowed. We do * this mainly to prevent reading arbitrary files. @@ -1969,3 +1983,55 @@ ResOwnerReleaseSnapshot(Datum res) { UnregisterSnapshotNoOwner((Snapshot) DatumGetPointer(res)); } + +/* + * CreateReplicationSlot() with the CRS_USE_SNAPSHOT option would be useful + * for testing, but regression tests cannot speak both replication and query + * protocol at the same time. This function can be used instead to create a + * snapshot for special tests of logical replication. + */ +Snapshot +create_test_snapshot(void) +{ + const char *slotname = "test_slot"; + const char *plugin; + LogicalDecodingContext *ctx; + Snapshot snap; + + /* + * XXX Hard-wired values are fine for the special test that needs this + * function. + */ + plugin = "test_decoding"; + + Assert(!MyReplicationSlot); + + CheckLogicalDecodingRequirements(); + + ReplicationSlotCreate(slotname, true, RS_TEMPORARY, + false, false, false); + + /* + * Ensure the logical decoding is enabled before initializing the + * logical decoding context. + */ + EnsureLogicalDecodingEnabled(); + Assert(IsLogicalDecodingEnabled()); + + ctx = CreateInitDecodingContext(plugin, NIL, true, + InvalidXLogRecPtr, + XL_ROUTINE(.page_read = read_local_xlog_page, + .segment_open = wal_segment_open, + .segment_close = wal_segment_close), + NULL, NULL, NULL); + + /* build initial snapshot, might take a while */ + DecodingContextFindStartpoint(ctx); + + /* Do what the function is called for. */ + snap = SnapBuildInitialSnapshot(ctx->snapshot_builder); + snap = CopySnapshot(snap); + FreeDecodingContext(ctx); + + return snap; +} diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index b8c01a291a1..9f0d60ebc1b 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -123,4 +123,5 @@ extern Snapshot RestoreSnapshot(char *start_address); struct PGPROC; extern void RestoreTransactionSnapshot(Snapshot snapshot, struct PGPROC *source_pgproc); +extern Snapshot create_test_snapshot(void); #endif /* SNAPMGR_H */ -- 2.47.3 --=-=-=-- ^ permalink raw reply [nested|flat] 486+ messages in thread
end of thread, other threads:[~2026-01-19 11:54 UTC | newest] Thread overview: 486+ messages (download: mbox mbox.gz follow: Atom feed) -- links below jump to the message on this page -- 2019-05-10 02:22 [PATCH v3 11/12] s/recommendable/recommended Justin Pryzby <[email protected]> 2019-05-10 02:22 [PATCH v5 12/12] s/recommendable/recommended Justin Pryzby <[email protected]> 2024-09-19 04:48 [PATCH v22 6/8] Row pattern recognition patch (docs). Tatsuo Ishii <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]> 2026-01-19 11:54 [PATCH 1/2] Demonstrate possible race conditions in logical decoding. Antonin Houska <[email protected]>
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